Regeneration systems for particulate traps for diesel engines have typically employed an auxiliary fuel burner unit which elevated the temperature of gases flowing through the trap to bring about carbon ignition and oxidation. One type is based upon the concept of bypassing exhaust gases during the regeneration cycle so that the filter or trap portion from which the exhaust gases have been removed can then be heated to an oxidation temperature for removing the particulates. In such bypass type of regeneration system, the total apparatus assembly usually requires a sizable number of components, including a trap (such as a ceramic honeycomb with alternately blocked channels), bypass valve and channel apparatus, a fuel pump and associated nozzle for delivering a well-atomized fuel spray, an air pump for supplying combustion air, ignition means for igniting the combustible fuel/air mixture for additional heat content to the exhaust flow, a control system to detect when to initiate regeneration based on the ratio of trap back-pressure to the clean trap back-pressure, and a control system to sequence the bypass of the exhaust flow, initiate heating, and turn off the ignition system when the regeneration is complete. Such an apparatus is shown in U.S. Pat. No. 4,167,852. This apparatus is complex and expensive because of the number of elements that make up the assembly. For example, the controls to detect when to initiate the regeneration and the controls to sequence the bypass of the exhaust valve, as well as the fuel pump, air pump, and the bypass valve and channel structure, are all rather expensive and complex items which place a heavy burden on the designer to assure reliability.
Electrical regeneration systems may tend to offer greater reliability and are known in the art. One such electrical system is disclosed, in U.S. application Ser. No. 573,277, now U.S. Pat. No. 4,544,388 (assigned to the assignee of this invention) wherein, in place of the fuel pump and nozzle, an electric heater is employed along with an air pump and a temperature control to ignite the particulates in the section of the filter that has been bypassed. Comparatively speaking, this apparatus requires unusually high power to heat the entire flow that goes through the filter portion; this requires a large, heated flow and accompanying complex controls to carry out the necessary heating. It would be desirable if such bypass system could be substantially reduced to a more simple structure to accomplish the regeneration without the necessity for such controls.
Another type of electric regeneration system is of the shifting heater type where a unitary filter trap is progressively and sequentially cleansed by either rotating an electrical heating element across the face of the filter, as shown in U.S Pat. No. 4,359,864, or the electrical heating elements are embedded in the ceramic filter itself and are sequentially energized to carry out a movable heating of different sections of the filter while a movable shield is carried across the face of the filter to block off exhaust flow in that portion of the filter in which the electrical elements have been energized, as shown in U.S. Pat. No. 4,276,066.
U.S. Pat. No. 4,359,864 is disadvantageous because it requires a full flow of exhaust gas through the entire filter at all times, and because the electrical heating element configuration, shaped as a rod which is rotationally scanned across the face of the filter element, much like the hand of a clock, has inadequate contact with the total gas flowing therethrough. The amount of electrical power required for such heating element must be great enough to heat the large exhaust gas flow which is passing thereacross and to raise such exhaust gas flow to a substantial temperature level permitting ignition of the particulates. This is a particularly inefficient method for bringing about the ignition of particulates since it is the temperature of the flow through the filter which brings about the ignition, such flow receiving its heat content from the electrical heating element. Thus, this patent demonstrates a disclosure which is not feasible because the electrical resistance heater must heat the total exhaust flow, which requires more energy than can be generated in conventional on-board resistance heater elements.
With respect to U.S. Pat. No. 4,276,066, the heating elements are embedded in the ceramic filter trap and encounter the problem of the thermal expansion of the heating element in the ceramic in the trap tending to crack the ceramic. With a cracked ceramic matrix, the trapping efficiency of the trap drops to very low levels. In addition, the resistance heater must be the sole mechanism for carrying out ignition of each and every particulate, since all exhaust flow through the energized elements is blocked off during such regeneration.
What is needed is a rotary type of electrical regeneration system which: (a) does not suffer from the problem of electrical heating elements embedded within ceramic; (b) does not require heating of the entire full flow of exhaust gas to bring about combustion of the particulates; and (c) will provide conjoint movement of a reduced, shielded, oxidizing gas flow and a segmental heater about the filter face.